This application proposes four integrated projects centered on male infertility and its biological basis. Project I will examine the role of inadequate intratesticular testosterone concentration in human male infertility. The hypothesis that reduced intratesticular testosterone concentration is responsible for some conditions of infertility will be tested directly in infertile men. Project II will examine the mechanism(s) by which testosterone protects germ cells from apoptotic death. A central hypothesis is that testosterone promotes the survival of germ cells via its effects on the Sertoli cell/germ cell Fas system, and through the Bcl- 2 family of cell survival/cell death proteins of germ cells. The results of Projects I and II should elucidate common principles involved in regulating spermatogenesis in rats and men. Project II focuses on how testosterone functions to regulate spermatogenesis through the androgen receptor (AR). The hypothesis will be tested that stage-specific regulation of AR gene expression in Sertoli cells is affected via the differential expression of endogenous transcription factors that bind to the AR promoter and enhance or repress transcription of the AR gene. The hypothesis that the AR is able to recruit endogenous co-regulatory proteins in Sertoli cells, and that these protein-protein interactions activate or repress AR transcriptional activity of genes required during spermatogenesis also will be examined. Thus Project III also provides a link between intratesticular testosterone (Projects I and II) and stage- specific gene expression (Project IV). Project IV examines the interactions between germ cells and Sertoli cells, critical for how spermatogenesis is coordinated and regulated. It will examine the function and regulation of stage-specific gene expression of analyses of Cyclic Protein-2, the pro-enzyme form of the cysteine protease Cathepsin L. It will be determined whether cis-acting elements in the CP-2/Cathepsin L gene and trans-acting factors in Sertoli cells interact in a stage- specific manner; and whether these regions are responsible for stage- specific transcription of the CP-2/Cathepsin L gene.